fix: transient Metrics view not found error flash in canvas table/pivot components

[nishantmonu51]

• On canvas page load, the table/pivot component briefly flashed a red Metrics view not found error before rendering: while the ListResources query was still in flight, validateTableSchema treated the momentarily-undefined metrics view spec as a hard error.
• validateTableSchema now takes the { metricsView, isLoading } query result and reports a loading state instead of an error while the fetch is in flight, matching the existing guards in the KPI, chart, and leaderboard components.
• CanvasPivotDisplay skips populating pivot state until loading completes, so column/time-dimension mapping is not computed from an undefined spec. While loading, the component falls through to the pivot's existing loading spinner; a genuine missing metrics view still shows the error.

Checklist:

• Covered by tests
• Ran it and it works as intended
• Reviewed the diff before requesting a review
• Checked for unhandled edge cases
• Linked the issues it closes
• Checked if the docs need to be updated. If so, create a separate Linear DOCS issue
• Intend to cherry-pick into the release branch
• I'm proud of this work!

[nishantmonu51]

@codex: review

[chatgpt-codex-connector]

Codex Review: Didn't find any major issues. 🎉

Reviewed commit: a42b2e0ef7

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